At the present there are in the U.S. alone upwards of 105,000 inadequate highway bridges. A majority of them are functionally obsolete while a lesser number of them are structurally deficient. The latter are defined as bridges which had to be restricted to light vehicles only or closed, while the former are identified as bridges which can no longer safely service the system of which they are an integral part. The problem is of such magnitude that the U.S. Congress intervened with a special Bridge Replacement Program pursuant to Section 204 of the Federal-Highway Act of 1970. In a recent report pursuant to that program the Federal Government estimated in May of 1977 that the replacement cost for the deficient bridges on a current dollar basis is approximately 23 billion dollars. The actual cost for replacing deficient or obsolete bridges is substantially higher, since not all such bridges were or could be inventoried. Informed estimates place the real replacement cost at around 30 billion dollars.
About two-thirds of these bridges are relatively small bridges having a span of 30 to 100 feet. Many, if not most of them are in remote locations, are accessible by narrow highways only, and are not within easy reach of the equipment, especially heavy cranes, needed for the erection of conventional bridges.
In view of the immensity of the U.S. bridge deficit attempts have been made to economize the construction of bridges. To the present, no bridge structure has been developed that can be reliably produced in large quantities, is readily, i.e. inexpensively transported to the construction site, and can there be erected, all in compliance with the bridge specifications of the American Association of Highway and Tranportation Officials (hereinafter AASHTO). Currently, efforts are being made to devise prestressed concrete bridge structures to fill the need. They have, however, several problems.
First, the great weight of concrete members requires special transportation and hoisting equipment. In many instances, the access to the construction site might be insufficient for such equipment, especially cranes with the necessary capacity of as much as 100 to 250 tons, unless the equipment is first dismantled. This greatly increases the overall costs for replacing an obsolete bridge.
Secondly, there is an apparent inability to organize and disperse manufacturing outlets in a sufficiently large number and over a sufficient geographic area so as to reduce the significant transportation costs for such heavy elements as concrete bridge structures.
Thirdly, present day technology makes it difficult if not impossible to adequately control the quality of prefabricated concrete so as to enable the construction of structurally efficient bridges, that is of bridges which are not excessively heavy so as to compensate for possible material defects.
In view of the foregoing problems surrounding National Bridge Replacement Program, the replacement of bridges presently proceeds at a very slow pace. Only a few thousand bridges within the Federal Bridge Replacement Program have been replaced over the last five years. At that rate it would require half a century or more to fully upgrade tha national bridge inventory. Of course, in addition to the replacement program new bridges for new roads and highways are required on an ongoing basis, thereby further increasing the demand for bridges.
A bridge structure which is not afflicted with the above summarized shortcomings of prior art structures is therefore urgently needed by this nation as well as on a worldwide basis.